This invention relates to a device for loading explosives in a seismic shot hole and to a method for its use.
More particularly, this invention is directed to an insertion tool for use with loading pre-packaged explosives into an uncased seismic bore hole.
Seismic prospecting is typically carried out by drilling a hole or a pattern of holes to a pre-determined depth. An explosive charge is loaded into the hole and is detonated using electric blasting caps. Reflected and refracted sonic energy is sensed by a geophone array placed in a pattern on the ground surface around the shot hole. The geophones are connected to a recording truck where both oscillograph and magnetic tape recordings are made. Analysis of the records thus made provides information as to the subsurface structure.
Shot hole depth is typically about 100 feet although the depth can vary depending upon the thickness of the surface weathered layer. Diameter of the drilled holes is maintained as small as possible in the interests of rapid drilling rates at as low a cost as possible. Typical hole diameters range as small as three and five eights inches.
Pre-packaged explosives are routinely used in seismic exploration. An explosive charge is packaged within a container or shell usually fabricated of plastic. The shell is of cylindrical shape, is standardly either two and one quarter or two and one half inches in diameter containing five pounds of explosive. Individual shells are arranged with a threaded male and a threaded female end to allow assembly of multiple shells into a single charge of desired explosive weight. The weight of explosive used in a single seismic shot will vary according to conditions but typically will be on the order of fifty pounds requiring ten containers or shells to be assembled end to end. Length of an assembled charge of this weight is on the order of fifteen to twenty feet. One such explosive container in common commercial use is described in U.S. Pat. No. 3,185,092.
A seismic shot hole is loaded by assemblying an explosive charge, priming it with an electric blasting cap, and inserting the charge into the hole. Because of the relatively narrow clearances between the charge containers and the hole wall, it is seldom possible to simply lower the charge to the hole bottom. Often the hole is at least partially filled with mud and it is not unusual for the hole to partially cave. Frequently, the charge must be pushed through these obstructions in order to properly place it at the bottom of the hole. A series of wooden poles, called stinger poles, is used for this purpose. The poles are usually ten feet long and are connectable end to end.
Occasionally a hole is so badly obstructed, by caving or otherwise, that the charge cannot be forced to the hole bottom. If the obstruction is located at a point where the charge top is more than a few feet into the hole, there is essentially no way that the charge can be removed from the hole. The only connection to the charge at this point is the wire leads to the electric blasting cap. Attempting to pull the charge out with these leads would either break the leads or pull the cap from the charge.
It is not permissable to leave the live charge within the hole. Consequently, the only choice presently available is to detonate the charge. If the charge is stuck relatively near the top of the hole, detonation causes surface cratering which requires backfilling. In summary, a stuck charge usually results in loss of both the shot hole and the charge and occasionally results in surface cratering as well.
In spite of the fact that this problem has long been recognized, the industry has failed to provide a solution for it.